The present invention relates to pulse arc welding methods. More particularly, the invention relates to improvements in pulse arc welding methods for welding using a current of a pulsive waveform.
In conventional welding employing a non-consumable electrode, i.e. a TIG (tungsten inert gas) pulse arc welding method, welding is conducted by using a current of a rectangular pulse waveform essentially as indicated in FIG. 1. The waveform of this pulse current is determined by four factors, i.e. a pulse current Ip, a base current I.sub.B, a pulse duration Tp and a base duration T.sub.B.
The advantage of the conventional welding method employing such a pulse current, as is evident from FIG. 1 is that a high current may flow for a short time between the electrode and a workpiece to be welded while the average current I.sub.M is maintained at a low value. Some advantageous features of the TIG welding method for welding by flowing a high current for a short time are that the arc is stable and highly concentrated, penetration is considerably deep and the bead is relatively wide.
In the conventional TIG pulse arc welding method, considerable experience and skill is required in setting the four factors, i.e. pulse current Ip, base current I.sub.B, pulse duration Tp and base duration T.sub.B, so as to achieve a desired pulse current waveform. This procedure is especially difficult for a beginner who would be required to conduct preliminary experiments to obtain the proper values. Thus, the conventional TIG pulse arc welding method disadvantageously necessitates complicated procedures.
In conventional welding employing a consumable electrode, e.g. MIG (metal inert gas) or MAG (metal active gas) pulse arc welding, the advantage of using a pulse current is that, even if an average current is lower than a critical current, a wire electrode can be finely broken up to form small droplets by means of an electromagnetic contraction force due to the pulse current and the small droplets can be "spray-transferred" to a base material. In the MIG welding operation, when the welding current is increased, the wire is finely broken up or spray-transferred to the base material when the welding current becomes higher than a predetermined "critical current" value. The critical current varies depending upon the material of the electrode wire, the diameter of the wire, the type of shielding gas used, the length of the wire extension, and so forth. As an example, the critical current varies as indicated by a broken line in a graph shown in FIG. 2 employing a wire of soft steel having a diameter of 1.6 mm, a shielding gas of argon +1%-oxygen and a D.C.R.P. of 6 mm of arc. The critical current (abscissa) is plotted vs. the number of particles per second and the volume of the particles per mm.sup.3 (ordinates). The average current means an average current value of the pulse current. When the peak current is set to exceed the critical current even if the average current value is lower than the critical current is indicated in FIG. 3, the wire can be spray-transferred to the base material. In FIG. 3, the critical current is indicated by a broken line, and the average current is indicated by a one dot chain line.
The waveform of the pulse current of this type has conventionally had, as indicated in FIG. 4, a pulse frequency f=1/T, a peak current value Ip, a pulse width .tau. and a base current I.sub.B. Accordingly, in order to select the peak value Ip, the pulse width .tau. and the base current I.sub.B to their proper values, considerable experience and preliminary experiments are required in the same manner as in the TIG pulse arc welding method.
When a welding operation is carried out with the conventional pulse arc welding method, it is necessary to set all of the above described factors every time the quantity of heat applied to the wire must be changed during the course of welding, which necessitates a very complicated control system. Particularly when it is necessary to control the quantity of heat applied to the wire in the up slope of the welding current at the start of welding and in the down slope of the welding current in a crater treatment at the end of welding, in the case of maintaining the bead shapes of the respective positions on a circumference of a stationary tube in all-attitude welding, in the case of obtaining a uniform and preferable bead shape when the gap, arc length and aiming position are varied over the entire length of the base material, and in the case of automatically arc welding by providing the end of a wire electrode with an oscillation or like motion in synchronism with the feeding motion of the end of the wire electrode, in the case of switching between a high current and a low current in a frequency pulse welding operation, a number of adjustment volume controls are necessary for setting the above-described four factors to obtain an optimum pulse waveform, thus resulting in excessively complicated manipulations and also resulting in a large-sized and expensive welding machine.